BTW, I hope there's a way to disable or tune this... Some CJK fonts seem
to be very small (when rendered at the same size as other characters),
and I think forcing them to be spaced with 2 * the width of the default
font would make CJK characters look weird, with tons of whitespace in
between each character....

Why would it look weird?

As I said, because sometimes the CJK font chosen for a given
point-size is very small compared to the ASCII font, the point where
each CJK character is almost as small as a single ASCII character.
[I didn't just make this up, I looked at the fonts being used for my
Emacs on several machines, and thought "hmm, how would this look if
CJK font-spacing was forced to double-width...?"]

Would you please provide some example characters and such a font to help
us make it better?
or If you had a linux environment, you can also try the fix and give
some hints.

I am a native CJK user and know well about Chinese and a little
Japanese/Korean, I really can not understand why shall we use such a
small font?

As I mentioned, such a small character already unreadable, font design
should follow some standards,
If I mean to use size 10, but the font give me a very small glyph like
size 5, these fonts is really a big problem.

Some katakana characters in Japanese may half width, that's OK. as I
said, it already be filtered out.
In emacs, Ctrl-h h shows a multilingual file, the Japanese string
"Japanese (日本語) こんにちは / ｺﾝﾆﾁﾊ" in katakana form is half width.

Like the author said, the terminal has rendered it that way for long.

Then the terminal probably does a better job of choosing a CJK font to
match the ASCII font, or simply uses a different ASCII font...
I guess the point is that there are multiple interrelated issues here:
character-spacing, but also font choice.

Yes, I prefer leave the font auto matching.

Ideally the best solution is to automatically choose good matching
fonts (and give the user powerful methods for overriding such
choices).

No such a good font, except use a real CJK monospace fonts, but its
English glyphs look weird.